326 MR. J. S. BUDGET! ON THE STEUCTUIiE 



haemal arches, together with the interspinous processes of the dorsal fin, become more 

 and more oblique, lying almost parallel to the notochord (PI. XXXIII. fig. G, d.sp., 

 l.sp.). Finally there lies upon the dorsal, and also upon the ventral, side of the notochord 

 a flattened mass of cartilage, which appears to be composed of the rudiments of several 

 neural and haemal arches (PI. XXXIII. fig. Q),f.n.pr.,f.v.'pr.). The notochord extends 

 beyond this incomplete cartilaginous tube, a bare filament embedded in the fin-rays of 

 the tail. 



At this stage there is no trace of any intervertebral cartilages, neither is there any 

 trace of a ring of cartilage enclosing the intervertebral regions of the notocliord. 



In older specimens of 9 cm. length considerable deposits of bone have appeared, 

 producing important changes in the structure of the vertebral column and its 

 processes. 



In the vertebral regions the cartilaginous bases of the neural arches and lateral 

 processes are still seen, though now surrounded by bone, deposited in the connective 

 tissue adjoining the notochord (PI. XXXIV. figs. 3, 4, 8, n.'pr., l.pr.). The ventral 

 cartilages have disappeared from the neighbourhood of the notochord. The bony 

 deposits which have enclosed the lateral cartilages and formed the transverse processes 

 have caused the proximal ends of the ventral cartilages to recede outwards, and these 

 are now found attached loosely to the underside of the transverse processes below their 

 attachment to the lateral bony ribs, which have now become formed in the connective 

 tissue of the septa between the dorso-lateral and ventro-lateral muscles (PI. XXXIV. 

 figs. 3, 4, 7, tr.pr.). The ventral cartilages themselves form the heads of the ventral 

 bony ribs which have now become formed at the junction of the septa between the 

 successive ventro-lateral muscles with the peritoneum (PL XXXIV. figs. 4, 7, v.])r.). 



The shifting outwards of the heads of the ventral ribs or haemal arches with the 

 development of the large transverse bony processes of the vertebrae has resulted in 

 allowing space for the greatly enlarged kidneys. 



It will be seen that the heads of these ventral ribs do not now lie quite in the same 

 transverse plane as the transverse processes, but crossing them diagonally project 

 towards the muscles in front of the transverse processes (PI. XXXIV. fig. 7). In the 

 full-grown Polypterus the heads of the ventral ribs articulate with the intervertebral 

 regions of the vertebral column, that is, between two successive vertebrae, and it might 

 be thought that this was their original place of attachment. It will, however, be 

 understood, from the above description of their mode of growth, that they are 

 originally attached to the ventral surface of the notochord in the sa.me transverse plane 

 as the bases of the transverse processes, that is, vertebral in position, and that they later 

 lose their connection with the notochord, become twisted across the ends of the 

 transverse processes, and with the subsequent development of the bony centra become 

 attached again to the vertebral axis, and are finally intervertehral in position. 



